The danger zone for autos is about 50 feet on down. You don't have enough altitude to spin up the blades if they have slowed at all.

Btw, negative 4 is fine for autoing. I prefer minus 9 to be available if i need it but rarely ever use it. the only time i do use that much negative pitch is if i think I'm going to overshoot and I want to lose altitude fast.

Try this on your next auto, bring the collective down first before you throw the T-hold switch. It's almost at the same time but with the collective stick movement slightly leading the T-hold switch. You be surprised how much rpm you lose between when you throw the switch and you pull the stick down. It takes time to recover from that. This will help minimize it.